One of the things I think makes Blood Money neat in respect to Hitman as a whole is the way they use Agent Smith.
Agent Smith is a CIA agent who finds himself stripped down to his boxers and needing to be rescued in every Hitman game. (As a sidenote, I find the amount of male bondage and torture in Hitman games inordinately great. I am sure there is some psychological angle of interest to that, given their target audience of STRAIGHT MALE GAMER.) They don't really go into how these things happen in the narrative, so Agent Smith comes off as a hapless failure of a spy and 47 treats him as such. I personally like to think that this is his specialty, though. I can't see why the CIA wouldn't get rid of him otherwise. Perhaps his allowing himself to be captured so often is a part of his missions. He’s obviously very hardy and can take a beating, although he starts to break down from it in Silent Assassin, taking to drinking on the job and becoming even more of an apparent liability.
Anyway, like Mei Ling, he never really served any purpose beyond needing to be rescued. He's somewhat more important than her in that rescuing him is never an optional objective. 47 is always tasked specifically to retrieve him, whereas his meeting Mei Ling in a mission happens incidentally.
In Blood Money, he has the usual "I have gotten myself into a situation and need help getting out" scenario. What's interesting is that this time, the means of rescuing him are much more convoluted. In prior games, it was always just a thing of finding him, there would be a cutscene where 47 would break him out of whatever restraints he's got, and Smith'd see himself out there.
Flatline, however, is much more complicated. He's being kept prisoner in a hardcore hoity-toity detox clinic where the patients either leave sober or dead. There is a morgue on the premises for this possibility. Smith, who was sent there to determine which of the three possible targets 47 was supposed to kill (and maybe also to get a handle on his alcoholism, ’cause he looks like hell compared to the last game), is being drugged and kept in the locked-down hospital wing.
Because of the whole sober-or-dead thing, ICA has provided 47 with a experimental serum that will make Agent Smith look dead. When you find him, you inject him with the serum then scram. The orderlies will then take Smith to the morgue. After doing the other shit you were assigned, you go to the morgue on your way out and give Smith the antidote.
Unlike his other appearances, though, Smith becomes important later on. In the second to last mission, the ICA is completely dissolved and Diana has gone into hiding. Your controller for this mission, to thwart a plot to kill the President by killing the assassin and the Vice President who ordered the hit first (yes rly), is none other than Agent Smith and his bag of rubies. A surprising turn of events indeed, given that Smith was always nothing more than a chew toy before. I note as well that while Agent Smith has hereto been treated as a Butt Monkey, his rescue mission afforded him some dignity this time, perhaps because later you have to accept working for him. They expand upon Smith's motivations as a character too, specifically his belief that 47 and he are friends of a sort and his devout patriotism. Although we already knew that by his star-spangled boxers.
That's all incidental, though. His real use is that the serum from his suddenly much more complex rescue method becomes a Chekhov's Gun. Diana uses the same serum to seemingly "kill" 47, then administers the antidote at his funeral.